The present invention relates to a method of producing an apertured covering sheet for an absorbent article such as a diaper, a sanitary napkin, an incontinence protector, or the like, wherein heated needles are caused to penetrate a sheet of material comprising at least one thermoplastic component, and wherein the temperature of the needles during penetration of the sheet of material exceeds the melting temperature of the thermoplastic component.
The invention also concerns an apertured covering sheet for an absorbent article manufactured in accordance with the method, and an absorbent article provided with a covering sheet manufactured in accordance with the method.
High demands on softness as well as dryness are put on fluid permeable covering sheets for absorbent articles of the kind which during use are intended to be in contact with the body of a user.
However, it has proven difficult to accomplish a fluid permeable covering sheet having a soft, textile-like surface which remains dry even after repeated wetting when the covering sheet is being used on an absorbent article.
In order to achieve a soft, textile-like covering sheet, it is common to use nonwoven materials. In order to more quickly lead fluid through the surface material down into a lower, absorbent material layer, it is common to perforate the material.
One such perforated nonwoven material is previously known through, for instance, EP 0,235,309. The perforated nonwoven material consists of a spunlace material having a high percentage of hydrophobic fibres. In a spunlace process, holes are formed in a fibrous material by ejecting very high pressure water jets against the material. The spunlace material is one of two layers in a topsheet-laminate and is intended to be the layer which during use is arranged closest to the user. The spunlace material consists of a higher percentage of hydrophobic fibres than the lower material layer in the topsheet. Thereby, the lower layer can drain liquid from the upper layer.
However, one problem with the described material is that holes which are formed by water jets become irregular both in shape and in size and exhibit fibres protruding from the edges of the holes, into the holes. Such protruding fibres diminish the area of the holes and will in addition act as wicks which transport liquid into the material between the holes by capillary action. The protruding fibre ends and the irregular shape and size of the holes considerably increase the risk of liquid remaining in the covering layer after wetting. Since a very small amount of liquid is sufficient for a surface material to be perceived as wet, this is evidently a considerable disadvantage with the known surface material.
A further problem with the described nonwoven material is that it is difficult to create a predetermined, well-defined hole-size. It is well known, for instance through EP 0,409,535, that the hole dimensions of a perforated material are of determining significance to obtaining an optimal inflow of liquid. For nonwoven material exhibiting some areas having a dense fibre structure and other areas having an open fibre structure, this implies that it is difficult to obtain a uniform hole size. This is due to the fact that the holes in the dense fibre areas are smaller, since they are surrounded by more fibres. Moreover, such an apertured nonwoven material exhibits a relatively low tensile strength, since the aperturing involves a decrease in the strength of the material. Since it is important that the material has sufficient strength so that no risk of breaking exists either in connection with the aperturing process, during production of the absorbent article, or during use of the finished absorbent article, naturally the decrease in the strength of the material which accompanies the aperturing is a problem.
In EP 0,214,608 a nonwoven material is apertured using hot needles which heat the nonwoven material to a temperature which is somewhat below the melting point of the material. The holes which are thus created in the material are surrounded by an edge exhibiting a densified fibre structure. The previously mentioned problems with varying hole sizes and reduced material strength are partially solved with a material which is apertured in this manner. However, the problem with liquid spreading in the nonwoven and staying in its fibre structure still remains. The denser fibre structure around the holes is intended to absorb liquid in order to transport liquid through the holes into a material layer below. However, there is a risk that a portion of the liquid is left in the denser hydrophilic fibre structure surrounding the holes. Further, liquid may spread horizontally in the plane of the nonwoven material in the fibre capillaries in the nonwoven material. Since, during use, the nonwoven material is in direct contact with the skin of the user such spreading of liquid is, of course, extremely inconvenient.
In SE 9601681-1, nonwoven material is apertured with hot needles which heat the nonwoven material to a temperature exceeding the melting temperature of at least one component in the material. Accordingly, the holes which are thus created in the material are surrounded by an edge which is at least partially melted. The melted hole edge reduces horizontal spreading of liquid via the fibre capillaries, in the plane of the nonwoven material. However, it is possible to further improve the fluid pervious covering layer as disclosed in SE 9601681-1 in order to obtain a cover exhibiting both high softness and high surface dryness. Other examples of sanitary napkins including at least one layer with a plurality of apertures are given in EP 0,165,807 and U.S. Pat. No. 4,690,679.
The problem with accomplishing a fluid pervious topsheet for absorbent articles, which topsheet is soft and comfortable against skin and still exhibits high surface dryness has been substantially removed by the present invention.
Accordingly, the invention provides an apertured covering sheet exhibiting high softness as well as dryness against the skin of a user.
This is accomplished in accordance with the invention by means of heated needles which are caused to penetrate a sheet of textile material comprising at least one thermoplastic component, wherein the temperature of the needles during penetration of the sheet of material exceeds the melting temperature of the thermoplastic component. The heated needles melt the thermoplastic component closest to the needles whereafter the needles are removed and the heated material is passed between at least one pair of compression rollers whereby the thermoplastic component immediately surrounding the apertures is smoothed out in the plane of the sheet of material so that a substantially smooth material surface is formed around each aperture. The thermoplastic component is then solidified.
One advantage with an apertured sheet of material produced in accordance with the invention is that it exhibits high smoothness in the plane of the material. This means that the risk of the edges of the apertures in the covering sheet rubbing against the skin of the user is practically eliminated. A further advantage with the apertured sheet of material is that the surrounding, substantially melted edge of the apertures to a higher degree can be relied upon to constitute a continuous, liquid impermeable surface. Due to the fact that the thermoplastic material surrounding the apertures in the textile material is in a molten or at least softened state when the material is passed between the compression rollers, the molten or softened thermoplastic material is pressed into and fills out cavities between optionally occurring non-thermoplastic fibres in the textile material.
A further advantage of thermally aperturing the material so that the structure surrounding the apertures melts, and thereafter passing the material through the nip of a pair of rollers is that a specific aperture size is obtained with a high degree of repeatability.
The optimal aperture size varies with the intended use for the covering sheet. Menses and urine, for instance, have completely different surface energies and different Theological properties, which means that the design of the covering layer must be adapted accordingly. However, in a covering material in accordance with the invention, the shape and size of the apertures deviate only to a minimal degree from the intended optimal shape and size which has been decided with regard to the intended use for the covering material. Consequently, in accordance with the invention, well performing covering materials can be created for a wide range of different absorption purposes with great accuracy and repeatability.
Another advantage with passing the material through the nip of a pair of rollers directly after the aperturing step so that a substantially two-dimensional, flat material structure is created, is that the material demands less space when transported or stored. Moreover, it is possible to manufacture very thin absorbent articles when using such a covering material.
In accordance with one embodiment of the invention, the needles are of different size in the thickness dimension which means that a covering sheet which is apertured with such needles exhibits apertures having different aperture sizes. In accordance with an advantageous embodiment, when the apertured covering sheet is used as a fluid permeable covering sheet for a sanitary napkin, the apertures are of two different sizes. The larger apertures exhibit a diameter which is between 2-4.5 mm and the small apertures exhibit a diameter which is between 0.1-2 mm. For the larger apertures which preferably have a diameter greater than 2 mm this implies that a comparatively large amount of fibrous material must be eliminated from the aperture itself during the aperturing procedure. A small amount of material is evaporated due to the heat to which the material is exposed by the hot needles. The remaining amount of material will mainly attach itself to the edge of the created aperture where it forms a fluid impermeable edge. The total length of the edge of the aperture, which is the same as the circumference of the aperture, increases exponentially with a factor of two with regard to the radius of the aperture. This means that the larger the apertures in the material are, the more material per length unit is gathered along the edge of the aperture than for the smaller apertures.
When using the apertured covering layer as a fluid permeable covering layer positioned closest to a user, the edges of the apertures may irritate the skin. The edges of the apertures may irritate the skin both if the apertures are arranged over all of the surface of the covering layer and if the apertures are arranged only within limited areas such as, for instance, within the area of the article which is expected to be initially wetted by body fluid. In order to eliminate the problem with thick material edges, the flat-rolling of the covering layer after the aperturing is essential. Furthermore, for reasons which are explained in the above discussion, the flat-rolling is particularly important for covering layers exhibiting relatively large apertures.
In accordance with another embodiment, the distance between the needles is different within different areas of the covering layer whereby the sheet of material after the aperturing step exhibits areas with different spacing between the apertures. The advantage with such an embodiment is that it is possible to have the apertures more closely spaced within the area which is expected to receive the greatest amount of fluid and further apart along the longitudinal and transverse outer edges of the article.
In accordance with yet another embodiment, the sheet of material is passed through the nip between the rollers together with at least one additional fluid pervious sheet of material whereby the melted component surrounding the apertures in the apertured sheet of material adheres to the additional sheet of material. After passage through the nip between the rollers, the melted component is caused to solidify whereby the additional sheet of material is laminated to the apertured sheet of material. This embodiment is advantageous since a particular laminating step is eliminated. Usually, sheets of material are laminated using adhesive or by welding, such as, for instance, ultrasonic welding. However, a problem in connection with adhesive lamination is that the adhesive often penetrates the covering sheet so that a portion of the adhesive will be in direct contact with skin during use. This problem is also more pronounced when using covering layers having a large open area. In accordance with this embodiment the problem that an adhesive coating often causes reduced fluid wicking and acquisition is also avoided. A further advantage is that the production cost is reduced since no adhesive or additional process-step such as a welding step is needed.
In accordance with one embodiment, the additional sheet of material consists of a hydrophilic sheet of nonwoven. The advantage with such an embodiment is that the hydrophilic sheet of nonwoven absorbs fluid from the apertured sheet of material. Consequently, the apertured covering is efficiently drained of fluid.
The invention further concerns a fluid pervious covering layer which is produced according to the method for use in an absorbent article such as a diaper, an incontinence protector, a sanitary napkin, or the like. Accordingly, the fluid pervious covering layer comprises at least one apertured textile sheet of material comprising at least one thermoplastic component. The apertured sheet of material exhibits a substantially two-dimensional structure having a plurality of apertures each surrounded by a substantially fluid impervious edge consisting of a substantially smooth plastic surface in the plane of the material.
The apertured sheet of material preferably consists of a nonwoven material but may, of course, alternatively consist of other textile materials. As has been previously mentioned, the smooth aperture edge is particularly advantageous for covering sheets which exhibit relatively large apertures, something that is common in sanitary napkins. However, such covering sheet are also suitable as apertured covering sheets for children""s diapers and incontinence protectors. It is extremely important that fluid permeable covering sheets for children""s diapers exhibit high softness and smoothness, since the skin of a child is sensitive and, moreover, is not protected by hair. Moreover, for incontinence protectors, particularly of the kind which is used by older people who are mostly bed-ridden, it is very important that the covering layer which is closest to the user does not irritate the skin since wounds which may arise take a very long time to heal.
In accordance with one embodiment, the fluid permeable covering sheet comprises at least two layers of material. One advantage with a further layer of material which is positioned between the absorbent core in an absorbent article and the upper material layer positioned closest to the user is that such a layer may prevent superabsorbent particles or granules which come loose from the absorbent structure from spreading further out of the article. In addition to the fact that a reduced amount of superabsorbent material will lower the absorption capacity of the absorbent structure, the granules may also cause skin irritation when they come into direct contact with skin. A further layer of material will also serve to mask fluid which has been absorbed into the article, whereby the visual impression of, for instance, menstrual blood in a sanitary napkin does not appear as prominent as when using a single layer of material.
Still another embodiment of a fluid permeable covering sheet produced in accordance with the invention comprises a first layer of material and a second layer of material wherein the second layer of material is more hydrophilic than the first layer of material. The advantage with such an embodiment is that the second layer of material, which is more hydrophilic, will absorb fluid from the first layer of material and thereby improve the surface dryness of the cover. The second layer of material, which is more hydrophilic, also acts as a wicking layer, resulting in a more even distribution of fluid in an absorbent core arranged inside the cover. A fluid distribution layer also makes it possible to achieve a higher level of exploitation of the total absorption capacity of the absorbent core, which reduces the risk of fluid leakage.
In accordance with one embodiment, the second layer of material is non-perforated. The advantage with a non-perforated second layer of material is that a non-perforated layer increases the tensile strength of the covering sheet. A high tensile strength in the covering sheet is of importance during production of the covering sheet itself, during production of the absorbent article and, finally, during use of the produced article. However, it is possible to use a second layer of material exhibiting perforations.
The invention also includes an absorbent article such as a sanitary napkin, a diaper, an incontinence protector, or the like, comprising an absorbent core enclosed within a cover, wherein at least a portion of the cover consists of a fluid permeable covering sheet. The covering sheet is characterised in that it exhibits a substantially two-dimensional, flat structure. In addition, the covering sheet comprises at least one layer of material exhibiting a plurality of apertures. Each of the apertures is surrounded by a substantially fluid impermeable edge exhibiting a substantially smooth plastic surface in the plane of the material. Accordingly, the material exhibits the softness and textile feel of a nonwoven material at the same time as the smooth plastic surfaces around the apertures create a material exhibiting similarities with a perforated plastic film with regard to the ability of maintaining a high surface dryness.